Telephone systems in the United States provide central offices for receiving signals from calling telephones within a particular radius such as one (1) to two (2) miles from the central office and for transmitting telephone signals to such telephones. The telephone signals from a calling telephone are then transmitted through long distances from such central office. The telephone signals then pass to the receiving telephone through a central office within a radius of one (1) mile to two (2) miles from such central office.
The telephone signals are transmitted long distance between central offices through optical fibers which have replaced other media previously provided for such purposes. The optical fibers have certain distinctive advantages over the lines previously provided. They allow a significantly increased number of signals from different telephones to be transmitted at the same time through the optical fibers. They pass the digitally-encoded signals with a higher accuracy than other media.
Various systems have been adopted to carry digitally-encoded signals for telephone, video, and data services. One of such systems now being adopted is designated as asynchronous transfer mode (ATM). This system is advantageous because it recognizes that generally signals travel in only one direction at any one time between a calling subscriber and a receiving subscriber. The system preserves bandwidth in the other direction so that a maximum number of different messages can be transmitted in such direction.
In ATM systems, cells are provided to transmit information between access multiplexers or terminals through central offices. Each of the cells contains headers identifying the calling and receiving stations and also contains a payload providing the information being transmitted and received. The cells pass from the calling telephone through the access multiplexers to a first central station. The cells then pass through the first central station and optical fibers to a second central station and then to the receiving access multiplexer. During the transfer of the cells to the central stations, the headers may be changed. These changes in the address indicate the path that the cell is following between each pair of central stations to reach the receiving telephone.
In the prior art, to reassemble cells into signals at the access multiplex, the header and the payload in each cell have been transferred to a control memory that processes the header to determine what path it came from and thus reassemble the signal based upon this path. This has created certain difficulties. For example, it has required the control memory to be relatively large, particularly since the memory receives the header and the payload. It has also caused the transfer to be slow, particularly since the header and the payload have to be processed and the payload is generally twelve times longer than the header.
There are other problems in the operation of the ATM systems of the prior art. These problems have resulted from the fact that access lines and central stations receive cells from a number of different sources. Each source may illustratively constitute a different one of the calling stations and may illustratively have a different rate of transferring cells into successive time slots. For example, one source may transfer cells into an access line in every third (3d) time slot and another source may transfer cells into an access line in every fourth (4th) time slot. In the past, a decision has been made after the transfer of a cell in each time slot as to which source, if any, is to transfer a cell into the next time slot. This system has been cumbersome and slow because the transfer has to be interrupted after the transfer of a cell into each time slot while a decision is being made as to the transfer, if any, in the next time slot.